Biosensing instruments are used for the detection of various analytes (e.g., glucose and cholesterol) in blood samples. For example, blood glucose meters are medical diagnostic instruments used to measure the level of glucose in a patient's blood, and may employ disposable sample strips having a well or reaction zone for receiving a blood sample. Some meters include sensor assemblies that determine glucose levels by measuring the amount of electricity that can pass through a sample of blood, while other meters include sensor assemblies that measure how much light reflects from a sample. A computer microprocessor of the meter then uses the measured electricity or light from the sensor assembly to compute the glucose level and displays the glucose level as a number.
An important limitation of electrochemical methods of measuring the concentration of a chemical in blood is the effect of confounding variables on the diffusion of analyte and the various active ingredients of the reagent. For example, analyte readings are influenced by the ambient temperature that surrounds the sample well or reaction zone. As with any electrochemical sensing method, transient changes in temperature during or between measurement cycles can alter background signal, reaction constants and/or diffusion coefficients. Accordingly, a temperature sensor may be used to monitor changes in temperature over time. A maximum temperature change over time threshold value can be used in a data screen to invalidate a measurement. Absolute temperature threshold criteria can also be employed, wherein detection of high and/or low temperature extremes can be used in a data screen to invalidate a measurement. The microprocessor of a glucose sensor can make a determination as to whether the temperature of the testing environment is within predetermined thresholds, and prohibit a user from running a test if accuracy would be negatively affected. It is important, therefore, that any temperature sensing elements of the glucose meter not be affected by heat generated within the glucose meter (e.g., by a backlight liquid crystal display).
The temperature sensing elements of the glucose meter should have access to the ambient temperature surrounding the meter. In view of the temperature sensitivity of the biochemical reactions that are interpreted by a biosensing device, ambient temperature values that are obtained by temperature sensors are directly used during the assessment of analyte levels in the sample. As a consequence, even relatively minor variations in sensed ambient temperatures can create fluctuations in biochemical readings and result in erroneous outputs. Because the outputs provided by the biosensing device is intended to influence the patient's decisions regarding, inter alia, dosing of medication, it is very important that erroneous readings be avoided. Thus, biosensing instruments should include means for avoiding erroneous outputs that result from inaccurate or misleading ambient temperature readings.
Various prior art instruments employ internal or external thermal sensors in order to acquire information about the ambient temperature (see e.g., U.S. Pat. No. 5,405,511; U.S. Pub. No. 2006/0229502), while other instruments attempt to control the temperature of the reaction zone, and still other devices attempt to obtain indirect measurements of blood sample temperature by use of complex algorithms that rely upon the use of an ambient temperature sensor in combination with AC admittance measurements (see U.S. Pat. No. 7,407,811).
While sensors that are sensitive to ambient temperature are capable of rapidly reacting to a temperature change and thereby provide timely information, under certain circumstances this attribute can have undesired consequences. For example, when a biosensing instrument that is normally held in a user's hand is placed on a tabletop, a rapid temperature change may occur that can bias subsequent biochemical readings until ambient temperature readings have stabilized. As for instruments that attempt to control the temperature of the reaction zone, if the biosensing instrument is battery-driven, it becomes impractical to control the reaction zone temperature as this requires too great a power drain from the instrument's battery. Furthermore, certain approaches, such as that described in U.S. Pat. No. 7,407,811 do not provide a universal solution to the problem of estimating ambient temperature; the approach described in that patent is designed for use with a specific glucose strip, and if the strip chemistry or strip geometry changes, the disclosed algorithm must be modified. There remains a need for temperature sensing systems that can overcome these problems and otherwise improve the accuracy of analyte measurements by biosensing instruments.